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Arfa Daily wrote:
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I've got an Integra DTR-5 (nearly identical to the Onkyo TX-DS 575)
thats in circuit protection mode (immediately goes into standby as soon
as the the power switch is pressed on, and opens the main power relay).

The most likely scenario is that it is detecting a DC offset on one of
the
channels, but if you are new to faultfinding, you will probably struggle
to
get to the bottom of the problem with any certainty that you can
confidently
switch back on, without finishing up with another pile of dead
components.
The reason that you read infinity on the speaker lives, is because these
are
connected to the output stages via the output relay(s), which will be
remaining open, if the unit is in protect.

Thanks for the concern. It's already a dead pile of parts, so I might
as well learn something new. Saw the relays. That explains that...
The receiver has separate front channel and center/surround channel amp
transistor boards. I think it's been narrowed down to the surround
board - when I unplug the surround board's rail power, the circuit
protection does not kick in. I tried to catch the offset voltages on
the surround board before the protection kicks in, but the DMM doesn't
register fast enough.

To check for DC offset, you need to measure at the output transistors'
midpoint, which will be at the junction of the two low value emitter
resistors (assuming that the amp uses conventional output devices, and
not
an STK hybrid). At idle, you should have zero volts, or very very close
to
zero, on this point. If you find any offset on any channel, then it's
likely
that one or other or both of the output transistors has failed, but that
is
unlikely to be the end of the story.

Thanks - I see that (.22 ohm middle tapped resistors after the amp
stages, but before the coils). I've got to get external voltage to the
surround board rails somehow. The rails are 45V, but would driving them
at 12V (two 12V batteries, one for each rail, to give 24V delta)
provide enough voltage for a diagnosis?

With a DC coupled amp, the problem can
be coming from virtually anywhere right back to the front end, and other
transistors besides the outputs, will likely be faulty. If it does use an
STK hybrid - and I seem to recall that Onkyos don't - then any DC offset
will normally be cured, without any other issues, by simply replacing it.
If
you don't detect any offset, then the problem is with the protect circuit
itself (not unusual), but sometimes, these monitor many parameters, and
can
be an absolute bitch to troubleshoot.

This receiver has the C5198 and A1941 transistors attached to the heat
sinks (no STK's that I can see). It's a 5 channel + sub pre-out, along
with separate two channel B speakers (quasi-multi room). The surround
output circuit is fed by a LC4966 chip, which is fed by an M62447SP
chip (that feeds the front and center amp circuits directly as well).
Data sheets show the LC4966 as a quad bilateral switch and the M62447SP
as a 6 channele electric volume chip. Hopefully it's something on the
transistor board. I can almost follow analog circuits, and get a
concept of whats going on... I'd love to power each individual channel,
but there is one feed to the entire surround board, and the rest are
PCB traces and hard jumpers soldered in place (which I suppose I could
unsolder, if thats what I'm supposed to do).

If you have a full-rail offset on all channels - say +45v - then look for
a
missing negative rail or vice versa.

Thanks - will check.

I don't mean to put you off, but honestly, if you are new to
faultfinding,
this kind of project is not something I would recommend to a beginner.
Most
of us who are involved in repairing this kind of gear daily, bypass the
power switching, and use a variac to bring up the supply slowly, so that
we
can measure what's going on (wrong !!) before all those shiny new
transistors fail again ... Good luck with it.

I appreciate the advice, and love a challenge. I wish I had a Variac (I
see many flavors on Ebay, for $10=$100+) or knew of someone locally
(Farmington, CT) that had one, but I'll have to figure out another way
to bring up the supply voltages gradually. It's been a good learning
experience so far, and if this works, I've got a bookshelf system the
lost its right channel after it was dropped that would be nice to use
in the bedroom, and another one for the basement exercise room. I think
every point you mentioned generated about 2 hours of googling,
schematics, and testing, but I'm more comfortable already, and feel
like I'm starting to get the concepts. Thanks again!

Andris

The very first thing I would now do, before going any further, would be
simple ohms checks on the output transistors, and those 0.22 ohm emitter
reistors that you found. You may well find shorted devices quite quickly.
The chances are that only one channel out of the 5 main ones, has failed.
This means that you probably have an identical channel that is perfectly OK,
and will serve as a reference channel for comparative ohms checks. If you
find shorted outputs, initially, just remove them, and re-power the unit.
You will probably find that it then comes up OK on the other channels. You
can then go ahead and replace the faulty transistors, but do not then
repower with full input voltage, as there will almost certainly be other
issues that will reblow the outputs.

In order to carry out further checks safely, you will need to get the rails
on the output stages down at a very low level. This is where I would be
using my variac, but in order to do this, it is first necessary to bypass
the main power relay. To do this, simply solder temporary wire links across
the relay switch contact pins. If you do not have a variac, a crude but
effective way of reducing the input voltage to the unit, is to string 40
watt light bulbs in series - say 3 initially -and in series with the power
input. If you feel that you will be going down this route, you might want to
give this a try before fitting new output transistors, just to see what sort
of levels you get, as this will depend a lot on the idle power consumption
of the unit.

Once you have established low rails - say +/- 10v - you can start making
voltage checks to ground between similar points on a working channel, and
the (possibly) repaired one. If there are any significant differences, chase
them down, and replace the faulty components. You may well be looking at
driver transistors, coupling resistors to the bases of the output
transistors, output transistor emitter resistors ( the 0.22's ) and possibly
pre-driver transistors. If all voltages look similar between the reference
and repair channels, and the mid-point volts are zero or very close, you can
reduce the number of light bulbs by one, to bring the voltage up a bit
higher. Always re-check all the voltages each time you increase the input
volts, and let it run for at least 5 minutes, checking the output transistor
temperatures, all the time.

Trust me, it probably won't be an easy ride, and it's not a very good
project to pick as a starter, but you seem to have the enthusiasm and at
least a basic understanding of what you're doing, and as you say, you
haven't got a lot to lose by having a go. Again, good luck ...

Arfa